1. Field of the Invention
The present invention relates to a magnetoresistive sensor for reading information from a magnetic recording medium by utilizing a magnetoresistive effect and a magnetoresistive head constituted by including the magnetoresistive sensor. Further, the present invention relates, particularly, to a magnetoresistive head for use in a hard disk drive capable of reading at high recording density and relates to a structure of a magnetoresistive sensor for increasing the sensitivity of reading signals at high speed and high density recording and for attaining high signal reproducibility to improve the quality and a manufacturing method thereof. In particular, the present invention relates to a structure and a manufacturing method of a magnetic domain control film disposed in a magnetoresistive sensor for improving the sensitivity of a magnetoresistive sensor and attaining high signal reproducibility. According to the present invention, a magnetic head of high signal quality and reliability can be provided and a hard disk drive of high performance with low error rate can be provided by use of the magnetic head.
2. Description of Related Art
A magnetic head for use in a hard disk drive (HDD) comprises a writing head for recording information as magnetization signals into a magnetic recording medium (hard disk) and a reading head (sensor) for reading signals recorded as magnetization signals in the magnetic recording medium. Electric signals are converted by the writing head into magnetized information and recorded in the magnetic recording medium, while the recorded magnetized information is converted by the reading head into electric signals and taken out. In recent years, a magnetoresistive head of reading magnetized information by utilizing a magnetoresistive effect has been developed, which can read weak written magnetized information and attain a remarkable improvement on the recording density to greatly contribute to the information industry.
The sensor portion of the magnetoresistive head is composed of a stack of magnetoresistive layers comprising a plurality of magnetic thin films and non-magnetic thin films. The structure of the stack of magnetoresistive layers of the magnetoresistive head includes several types, which are classified, for example, into an AMR head, a GMR head and a TMR head based on the principle of magnet resistivity to be used. Input magnetic field information entered from the magnetic recording medium to the reading head is taken out as the change of voltage by utilizing the AMR effect (Magnetoresistive effect), GMR effect (Giant Magnetoresistive effect) and TMR effect (Tunnel Magnetoresistive effect), respectively.
The stack of magnetoresistive layers of the magnetoresistive head mainly comprises a magnetic layer referred to as a free layer that receives the input information magnetic field from the magnetic recording medium to conduct magnetization rotation, a pinned layer the magnetization direction of which is fixed by a coupling magnetic field of an anti-ferromagnetic body, and a non-magnetic layer put between them. Since the electric resistance of the stack of magnetoresistive layers changes in accordance with the change of the relation between the magnetization direction of the pinned layer and the magnetization direction of the free layer, when current is being supplied to the stack of electric resistant layers, a change in voltage according to the direction of the magnetization rotation of the free layer is generated and the direction of the magnetized information given to the free layer can be judged by observing the change in voltage. As described above, the reading head portion of the magnetoresistive head has a structure of functioning as a magnetic sensor by utilizing the magnetoresistive effective of the stack of magnetoresistive layers.
Since the magnetization direction of the pinned layer in the stack of magnetoresistive layers lowers the output signal intensity and allows the signal output to fluctuate when it is changed by the input signal magnetic field or other external magnetic field, the magnetization direction has to be fixed strongly by the coupling magnetic field of the anti-ferromagnetic body. For this purpose, an MnIr alloy thin film or MnPt alloy thin film having a strong coupling magnetic field is selected as the anti-ferromagnetic film, while a Co alloy film formed of a material having an intense coupling magnetic field or a stack of thin films thereof is selected as the pinned layer ferromagnetic film, and conditions for increasing the coupling magnetic fields are selected also for the thin film forming conditions. On the other hand, it is necessary that the magnetization of the free layer reacts sensitively to a weak external input magnetic field and has high reproducibility of the magnetization curve in accordance with plus, minus and zero of the external input magnetic field. For this purpose, an NiFe-Parmalloy alloy thin film or a Co series alloy soft magnetic film of excellent soft magnetic property, and a stack of thin films thereof are often used generally as the free layer. Also for the same purpose, materials and layer structures of the stack of magnetoresistive layers, as well as conditions and methods of manufacture have been studied and improved.
On the other hand, the Parmalloy thin film or the Co alloy thin film as the soft magnetic material used for the free layer are used in the sheet-like shape and they are used under application of induced magnetic anisotropy for attaining stable magnetization state even in a case where external input magnetic field is not present. However, it has been known that the soft magnetic thin films have no simple magnetic domain structure but form magnetic domain structures depending on the film thickness or the shape of the sheet. In particular, it has been known that circulation magnetic domain structures are formed at the end of the sheet-shape to generate disturbance in the magnetization direction, and the disturbed magnetic domain structures are changed by external magnetic fields. The noise generated due to by the change of the magnetic domain structures is referred to as Barkhausen noise which is generally known. In order to avoid the noise of the above mentioned type, a uniform bias magnetic field has to be applied to a sheet-like free layer to form a univalent magnetic domain structure so as not to generate circulation magnetic domains in the free layer. For this purpose, a permanent magnet is disposed at the end of the stack of magnetoresistive layers thereby generating and applying a bias magnetic field uniformly to the free layer. The system is known as a hard bias system and put to practical use. Further, it has been known an exchange bias system of forming an anti-ferromagnetic film on the free layer and applying a bias magnetic field by utilizing the exchange coupling between the free layer and the anti-ferromagnetic film. The hard bias system has been utilized for commercial use.
In the hard bias system, as shown, for example, in Japanese Patent Laid-open No. 3-125311, a magnetic domain control film comprising a magnet film is disposed on both ends of a free layer. The exchange bias system is a method of laminating an anti-ferromagnetic film on both ends of a long free layer and utilizing the exchange coupling between the anti-ferromagnetic film and the free layer as disclosed, for example, in U.S. Pat. No. 4,663,685.
Accordingly, the magnetoresistive sensor generally comprises, mainly, a stack of magnetoresistive layers in which a pinned layer or a free layer, or a pinned layer and a free layer are cut into a sheet-shape, and a magnetic domain control film having a permanent magnet film disposed on the end of a free layer cut into a sheet-shape for the magnetic domain control (hard bias system), or a magnetic domain control film of a system disposing an anti-ferromagnetic body on a free layer (exchange bias system), and an electrode film layer for supplying current to the stack of magnetoresistive layers.
A magnetoresistive sensor of the hard bias system is manufactured by a step of forming a stack of films for forming a stack of magnetoresistive layers, a step of coating a sheet-like resist which is well-defined for the size for forming a stack of magnetoresistive layers into a sheet-shape, a step of fabricating the stack of magnetoresistive layers into a sheet-shape, a step of forming a magnetic domain control film at the end of the free layer fabricated into the sheet-shape, a step of forming an electrode film layer, and a step of removing the resist coated for forming the shape. The magnetoresistive sensor of the exchange bias system is manufactured, for example, by a step of forming a stack of films for forming a stack of magnetoresistive layers, a step of forming an anti-ferromagnetic film on the surface of a free layer, a step of coating a sheet-like resist which is well defined for the size for forming a track width, a step of forming an electrode film layer, a step of removing the coated resist for forming the shape, a step of removing a portion corresponding to the track width of the anti-ferromagnetic layer, and a step of forming a protection film. In the case of the exchange bias system, the exchange coupling force applied to the free layer is weak and the track width has not yet been decided sufficiently in the process described above. In view of the present situation, developments of the material and structure for giving an intense coupling magnetic field and a manufacturing method have been required. Accordingly, the magnetoresistive sensor of the hard bias system is actually applied for commercial use at present.
As the permanent magnet film used for the magnet domain control film of the hard bias system, Co alloy series materials are used and those with addition of a Pt element have often been used. The Co series alloy thin films have hexagonal closed packed (HCP) crystal structure as the crystal structure and it has been well known that they have strong crystal magnetic anisotropy in the direction of the C axis and good permanent magnets showing high coercivity can be obtained easily. Further, addition of the Pt element to the Co series alloys increases the crystal magnetic anisotropy to show higher coercivity. It is further known that use of Cr or Cr alloy underlayer for the underlayer of the Co alloy thin film enables to control the crystallographic orientation of the Co alloy thin film by the hetero-epitaxy growing mechanism, thereby easily providing a permanent magnet film having higher residual magnetic flux density, coercivity and squareness. The techniques described above have been developed for the magnetic recording media.
Those used at present are magnetic domain control films of a stack structure of CoPt series alloy/Cr underlayer and they exhibit coercivity of about 2000 Oe and squareness of 0.8 or more. Further, there are those capable of providing coercivity of 3000 Oe or more by improvement of the material and optimization of the manufacturing conditions. The permanent magnet film of the magnet domain control film requires high coercivity since the magnetization state should not be changed by a signal magnetic field or an external input magnetic field. Since the input magnetic field is estimated to be about 600 to 800 Oe, it is possible that the coercivity of 1200 Oe, that is, at least 1.5 times thereof is necessary. Further it is possible that high values are required for the squareness and the coercivity squareness of the magnetization curve. When the squareness of the magnetization curve lowers, the residual magnetization lowers failing to provide a desired bias magnetic field efficiently to the free layer. The bias magnetic field applied to the free layer can be adjusted by changing the magnetic flux density of the permanent magnet film and changing the thickness of the permanent magnet film, while keeping high squareness, and optimization has to be done.
Generally, to conduct the magnetic domain control to the free layer in the stack of magnetoresistive layers, a magnetic field higher than a certain bias magnetic field has to be applied to the free layer. However, if the bias magnetic field is excessively intense, since the ferromagnetic body at the end of the free layer does not operate even when the signal magnetic field is inputted, a phenomenon of lowering the output occurs. For this reason, it is necessary to optimize the residual magnetic flux density or the film thickness of the permanent magnet film of the magnetic domain control film. Generally, the residual magnetic flux density is adjusted by changing the saturation magnetic flux density, that is, optimizing the alloy composition of the Co series alloy thin film. Further, the film thickness can be easily adjusted by changing the forming conditions and changing the forming time.
A sensor structure of a magnetoresistive head of a hard bias system in an existent structure is to be described with reference to FIG. 3. FIG. 3 is a schematic view showing a magnetoresistive sensor portion of a magnetic head taken along the cross section of the flying surface thereof. A stack of magnetoresistive layers is formed on a lower gap layer 2 formed on a lower shield 1. The lower gap layer 2 comprises a highly insulative material, mostly, an Al2O3 film. After forming an underlayer 3 for a stack of magnetoresistive layers, an anti-ferromagnetic layer 4, a pinned layer 5, a non-magnetic layer 6, a free layer 7 and a protection layer 8 successively on the Al2O3 gap layer 2, a resist is coated and the stack of magnetoresistive layers is fabricated into a sheet-shape by using the method of dry etching. In this process, the stack of magnetoresistive layers is dry etched as far as a lower portion thereof, that is, to the lower gap layer 2. The free layer width Twf as the track width is formed in this process. Subsequently, a magnetic domain control underlayer 10, a magnetic domain control film 11 and an electrode film 12 are formed continuously, and then the resist is peeled. Subsequently, an upper gap film 13 and an upper shield film 14 are formed. Among the forming steps described above, a method of forming the multi-layered thin film or forming the track width adopts a dry process, and the thin film is formed by a method usually referred to as sputtering, while the track width is formed by a method of ion milling. Further, when the stack of magnetic domain control layers 11 and the electrode film 12 are formed after forming the track width, they are formed by utilizing sputtered particles with stronger directionality by applying the ion beam sputtering method thereby optimizing the shape and the deposition of the magnetic domain control film 11.
The track width of the magnetoresistive head manufactured in accordance with the manufacturing method has been narrowed in recent years, and a resist refining technique or a technique of narrowing the size of the free layer has been developed. As shown, for example, in the Patent Document 1, application of a technique for forming a resist shape by electron beam exposure, improvement in the angle (α) at the end of the free layer to 45° or more by considering the resist shape, or a consideration of decreasing the gap between the free layer and the magnetic domain control film has been taken in order to improve the magnetic domain controllability.
However, in the course of progress of high recording density in recent years, narrowing for the lateral size of the free layer is required and, it is found that when the size is reduced to 200 to 100 nm or less, there is a limit in the existent stack structure of magnetic domain control films and that improvement in the manufacturing method is still insufficient.
Output Lowering, Formation of Dead Area and Output Fluctuation
Generally, reading head output intensity is in a substantially linear relation with the track width thereof and, when the size of the track width is narrowed, output is lowered in accordance with the extent of narrowing. By the way, it is a well-known fact that the output lowers more than the lowering of the output caused by the narrowing of the track width when the track width is about 300 nm or less. This is because a dead area not generating magnetization rotation is formed at the end of the free layer by the intense bias magnetic field applied by the magnetic domain control film to form a portion not causing magnetization rotation and not contributing to the output. As a result of various experiments and simulations, the size of the dead area is as large as about 60 to 80 nm being converted as the track width. Accordingly, while the lowering of the output is about 20% at the track width of 300 nm, the lowering of the output is as much as 60% at the track width of 100 nm and the output can be obtained scarcely.
It has been well-known that the track width size of the dead area depends on the intensity of the bias magnetic field generated from the magnetic domain control film and the output is improved when the bias magnetic field is decreased. However, when the bias magnetic field is decreased, reproducibility of the output waveform becomes poor because of insufficient magnetic domain control for the free layer and fluctuation of the output waveforms is generated, as well as this results in phenomenon of generating irregular Barkhausen noise or irregular noise after operation of the writing head. When the noise of this type is generated, an error rate of reading of the magnetic recording information increases and the head can no longer be used.
In the prior art, it is probable that the bias magnetic field depends greatly on the shape of the magnetic domain control film at the end of the stack of magnetoresistive layers and the amount of residual magnetization (Brt: product of residual magnetic flux density Br and the magnetic domain control film thickness t). With the view point described above, to optimize the bias magnetic field for suppressing noise or fluctuation of the output waveforms, three methods are conducted for optimizing the bias magnetic field, i.e., making the angle at the end of the stack of magnetoresistive layers abrupt, controlling the Co alloy composition used as the magnetic domain control film to control the saturation magnetic flux density Bs of a Co alloy thin film, and controlling the thickness of the Co alloy thin film used as the magnetic domain control film, thereby controlling the amount of residual magnetization (Brt) to control the effective bias magnetic field applied to the free layer.
As a result of an experiment, it has been made apparent that an increase of the residual magnetic flux density of the magnetic domain control film and a decrease in the thickness of the magnetic domain control film at the end angle of the stack of magnetoresistive layers of 60° is effective in increasing the bias magnetic field applied to the free layer and decreasing the residual magnetization amount but it has found that the effect is limited. In the experiment described above, in a case of a free layer with 100 nm in track width, fluctuation of the output waveforms was generated and irregular Barkhausen noses were generated at the amount of residual magnetization (Brt) of the magnetic domain control film of about 25 Tnm or less. The track length of the dead area of the free layer in this case was 60 nm and the length of the dead area could not be decreased further. Accordingly, it has been found that the bias magnetic field cannot be optimized completely and there is a limit to the improvement of the head output characteristics by this system.
Problem of Shape
The cause includes a problem with the shape of the magnetic domain control film disposed at the end of the stack of magnetoresistive layers. FIG. 4 is a schematic view showing the shapes of, and the positional relationship between the magnetic domain control film and each of the free layer, the pinned layer and the shield film, as well as the state of magnetization. FIG. 4(c) corresponds to FIG. 3 for the existent structure and FIGS. 1, 2 showing the structure of the present invention correspond to FIGS. 4(b) and 4(a), respectively.
The method of decreasing the amount of residual magnetization of the magnetic domain control film can include a method of reducing the thickness of the Co alloy thin film of the magnetic domain control film. FIG. 4(c) shows the shapes of, and the positional relationship between the magnetic domain control film 11, and each of the free layer 7 and the pinned layer 5. When the thickness of the Co alloy thin film is reduced, the thickness is reduced at the inclined portion of the end of the magnetoresistive element and a step is formed in the film shape thereat and, when magnetization H is directed to the track width, a demagnetizing field Hd is generated in the inclined portion to weaken the effective bias magnetic field. It is usually intended to apply the bias magnetic field only to the free layer 7, but the magnetic field is dispersedly applied to the pinned layer 5 and the shield layer 1, in the shapes shown in FIG. 4(c) and it cannot be said to be a structure for appropriately applying the bias magnetic field to the free layer. Further, the magnetic domain control film formed at the inclined portion of the end of the stack of magnetoresistive layers has a shape of a thin film inclined from the magnetizing direction. In addition, it is probable that intense positive and negative magnetic charges are generated on the inside and the outside thereof to form the demagnetizing field Hd inside the ferromagnetic body of the inclined portion. The demagnetizing field weakens the bias magnetic field intended to generate and no appropriate magnetic field can be applied to the free layer 7. Further, the angle of inclination (α) for the inclined portion at the end of the stack of magnetoresistive layers acts to weaken the magnetic charge density to weaken and disperse the bias magnetic field.
Further, the magnetic domain control film is formed by using a lift-off resist such that the thickness of and a portion near the top end of the magnetic domain control film 11 is generally reduced, and the top end formed thinly has a shape covering the upper surface of the free layer. It has been known that when the thickness of the Co alloy magnetic thin film is reduced to as thin as several nm, the magnetic characteristics thereof are lowered and the film becomes thermally instable. Accordingly, it is probable that the top end of the magnetic domain control film 11 also acts to disturb the bias magnetic field to be applied to the free layer 7.
It is considered that the magnetic domain control bias magnetic field should be applied to the free layer 7 and a magnetic field of higher intensity should be applied to the end of the free layer by a magnetic film whose vertical position is aligned with that of the free layer. In this case, it is expected that the bias magnetic field is applied more appropriately to the free layer 7 by adopting the structure of FIG. 4(b), that is, a structure as shown in FIG. 1 in which the vertical position of the free layer 7 is aligned with that of the magnetic domain control film 11.
Problem of Magnetic Characteristics
However, when a Co alloy magnetic thin film having a Cr underlayer used as the magnetic domain control film 11 is formed on an MnPt alloy thin film, MnIr alloy thin film, or on CoFe or NiFe used as a pinned layer of the stack of magnetoresistive layers, the magnetic characteristics thereof are deteriorated and the bias magnetic field cannot be applied to the free layer 7. That is, when a Co alloy thin film (magnetic domain control film 11)/Cr underlayer (magnetic domain control film underlayer 10) is formed on the thin film of the material used for the pinned layer 5, it resulted in a thin film having characteristics that the coercivity and squareness of the magnetization curve are lowered and the amount of residual magnetization cannot be maintained. As a result of various studies, this is attributable to that fitting is poor between the crystal system of the material constituting the stack of magnetoresistive layers and the crystal system of the Co alloy thin film/Cr underlayer (magnetic domain control film 11/magnetic control domain film underlayer 10) of good magnetic characteristics and, accordingly, a hetero-epitaxy mechanism results in a crystal structure of lowering the magnetic characteristics of the Co alloy thin film/Cr underlayer thin film when the Co alloy thin film/Cr underlayer are formed on the crystal system of the material constituting the stack of magnetoresistive layers.
While most of the layers of the stack of magnetoresistive layers are face-centered cubic (fcc) system polycrystal thin film, the Cr underlayer (magnetic domain control film under layer 10) used as the magnetic domain control layer is a body-centered (bcc) polycrystal thin film, and the Co alloy magnetic film (magnetic domain control film 11) is a hexagonal closed packed (hcp) polycrystal thin film. The Cr underlayer (10) used as the underlayer for the Co alloy magnetic film is used for controlling the crystallographic orientation and the crystal inner strains of the Co alloy magnetic film hexagonal systems by the hetero-epitaxy crystal growing mechanism and, as a result, a Co alloy magnetic film having high coercivity and high squareness can be obtained. On the other hand, when the Cr underlayer and the Co magnetic film are formed on the face-centered-cubic lattice magnetoresistive element, the hetero-epitaxy crystal growing mechanism exerts between the stack of magnetoresistive layers and the Cr underlayer and, as a result, gives an undesired effect on the crystallographic orientation and the lattice strain of the Cr underlayer and the Co alloy magnetic film, to deteriorate the magnetic characteristics. It has been found that the thin film 5 of Co alloy thin film/Cr underlayer less shows good magnetic characteristics on the thin film constituting the stack of magnetoresistive layers.
On the other hand, when the crystal structure of the stack of magnetoresistive layers is formed into a better face-centered cubic (fcc) crystal structure, the magnetoresistive characteristics are improved. It has thus been found that the crystal structure of the stack of magnetoresistive layers cannot be changed.
The fact that the Co alloy thin film on the Cr underlayer cannot provide good magnetic characteristics on the thin film constituting the stack of magnetoresistive layers also shows that the portion on the inclined surface of the Co alloy magnetic domain control film at the end of the stack of magnetoresistive layers in the existent structure is in a crystal state of deteriorated magnetic characteristics. It is estimated that deterioration of the characteristics at the top end of the magnetic domain control film induces lowering of the bias magnetic field and disturbance of the bias magnetic field and it is probable that this constitutes a cause of deteriorating the magnetic domain control film properties.
The problem with the deterioration of the magnetic characteristics at the inclined portion cannot be solved even adopting the system of increasing the thickness of the Cr underlayer and aligning the vertical positions of the free layer and the magnetic domain control film as described, for example, in Patent Document 2. Further, in a case where the thickness of the Cr underlayer is increased, the thickness of the underlayer at the inclined portion of the end of the stack of magnetoresistive layers is increased to also result in a problem that the gap distance is enlarged between the free layer and the magnetic domain control film.
Accordingly, it has been found that when it is intended to form the structure by aligning the height between the free layer of the stack of magnetoresistive layers and the Co alloy thin film of the magnetic domain control film, the crystallographic orientation of the Co alloy thin film cannot be optimized and the magnetic characteristics as the permanent magnet layer are deteriorated, so that no appropriate bias magnetic field can be applied to the free layer.
That is, in the prior art, it is difficult to apply an appropriate magnetic field to the free layer while maintaining the magnetic characteristics of the magnetic domain control film. Further, the bias magnetic field is applied not only to the free layer but also to the pinned layer and, also in view of a strict consideration, the end of the pinned layer also undergoes the bias magnetic field of the magnetic domain control film to incline the magnetizing direction of the pinned layer, thus forming a dead area.
It is an object of the present invention is to provide, in a magnetoresistive sensor adopting a hard bias system, a structure capable of applying an appropriate magnetic field from a magnetic domain control film to a free layer, by controlling the crystal structure of a Co alloy magnetic thin film used as the magnetic domain control film and, forming and disposing the Co alloy magnetic thin film in an appropriate shape at the end of the stack of magnetoresistive layers. As a result, it is possible to decrease the dead area of the free layer, improve the signal output and, further, decrease the Barkhausen noise, output fluctuation and asymmetry in the output waveforms, thereby improving the signal quality during high recording density.